Over the last few years the possible role of calcium in cell death produced by a number of toxins has been emphasized. Several studies have suggested that cell death is mediated through influx of extracellular calcium, although others disagree. Little attention has been given to the role of changes of intracellular, ionized calcium early after toxin administration. This project will examine the role of intracellular calcium in initiation of cytotoxicity produced by halogenated hydrocarbon hepatotoxin exposure. Halogenated hydrocarbons contaminate our drinking water and certain self-contained atmospheres. CCl4, CHCl3, and 1,1-dichloroethylene inhibit a calcium pump in the liver endoplasmic reticulum (ER) and release a pool of calcium from the ER. This project will examine suspensions of hepatocytes with fluorescent dyes that bind Ca2+ (QUIN-2 and MAPTAM) and with [45Ca2+] for evidence that ionized calcium in cytoplasm is increased and is important in initiation of cytotoxicity. The project will focus on the role of calcium pumps in the ER and plasma membrane in this process. The project will study mechanisms by which increased cytoplasmic calcium can initiate cytotoxicity. The long term objectives of the project would extend these studies from one class of hepatotoxins to other chemicals and drugs that produce hepatotoxicity.